So I'm slowly researching an article about stuff to hang onto camera lenses, and I come across references to FOCAL REDUCERS. Apparently these are well-know in the astro realm. Maybe I'm just not spaced enough... Anyway, I find little info about their significance and application. Perhaps some astrophotographers here can tell us about FR's. Here is what I think I know now, and what I'd like to know:
A Focal Reducer is a back-of-the-lens add-on like a TeleConverter, but with just the opposite effect. A TC increases focal length and decreases effective aperture; an FR *decreases* focal length and *increases* apparent aperture. So a 0.5x FR mounted on a 1000mm f/10 telescope turns it into a 500mm f/5 'scope -- a wider visual field, and it lets in more light. And this raises questions.
* What are the limitations? Is there a minimum focal length to which an FR can be applied? Can I mount my 24/2 on one to make it a 12/1?
* What are the implications? Is the camera-lens register (flange focal distance) affected? Is distortion introduced? Is IQ lowered noticeably? (The picture's IQ, that is -- my IQ is low enough already.)
* What is the construction? Can I just take an ordinary 2x TC, flip the optical element over, and turn it into a FR? Or must I buy one?
* On eBay I see various 0.5x and 0.7x etc FR's in 1.25in and 2in sizes, costing US$20-100+. I can probably adapt a 2in body to an M42 or PK mount. Are any of the cheap offerings any good, or just the usual optical trash?
* Should those of us NOT doing astro shooting even bother thinking about FR's? Have I just opened a worthless can of worms?
Please enlighten us (and me), oh Masters of the Skies!
EDIT: Yes, I gargled for
FOCAL REDUCER LENS and the first hit is
this article. I'm just fatigued and I'm having trouble digesting it. Simplification is welcomed!